Can I still get a large Billy goat here as they are rfh? Also do I need both deterrent flowers in two buckets to get the large Billy goat?

The RFH buttercups don't attract anything, so you'll need to start again with a small buttercup.

The bright side is that the big billy will still take the RFH buttercups.  Just make sure they are to the left of the FG one.

As for deterrents, that's up to you.  Some use them and some don't. 

The big billy goat will not come to the RTH flower.  If you had a fully grown big buttercup, and kept it to the right of the RTH ones, the big billy will come to the fully grown flower, but would take the RTH one.  You'd then still have the fully grown flower to attract again - a second one would take the second RTH plant, the third would take the fully grown one.
 
  I find it better to use both deterrents.  Unfortunately, you won't have room if you keep the RTH buttercups in your garden, which could make getting the big goats tricky.

Thank you for the help